1. Field of the Invention
The invention relates to an electropneumatic device for regulating the speed of a motor vehicle.
2. Description of the Prior Art
In a known device for regulating the traveling speed of a motor vehicle, an electric regulator is provided which compares the actual speed of a motor vehicle with a traveling speed preset by the driver. The regulator provides a signal indicative of the deviation of the two traveling speeds to a control stage. The control stage controls a pneumatic adjusting drive which acts upon the accelerator. The adjusting drive consists of a chamber which on one side is closed by means of a rolling diaphragm which is connected with the accelerator. The control stage associated with the adjusting drive comprises two magnetic valves mounted in the chamber wall of the adjusting drive, which can be controlled by the electric regulator. The chamber of the adjusting drive can be connected by way of one of the magnetic valves with the ambient air, i.e. the air surrounding the motor vehicle, and by way of the other magnetic valve with the carburetor intake pipe and thus be acted upon by the vacuum prevailing in the carburetor intake pipe. When the actual traveling speed of the motor vehicle exceeds the traveling speed set by the driver, the chamber is ventilated, and the adjusting drive actuates the accelerator toward a speed decrease. When, on the other hand, the actual traveling speed is lower than the preset speed, the other magnetic valve is actuated, and the chamber is acted upon by a vacuum, whereby the accelerator is adjusted toward a speed increase.
Such a structure of the control stage of the regulator has considerable disadvantages. For instance, an electric regulator with two signal outputs must be used for controlling the two magnetic valves. In comparison with an electric regulator with only one signal output, this results in an increased expenditure for circuits and structural parts and thus in increased production cost. In addition, because of vacuum variations in the suction pipe, the lift variation of the rolling diaphragm depends on the deviation of the actual traveling speed from the peset traveling speed as well as on the magnitude of the prevailing vacuum. In order to eliminate this dependency of the diaphragm lift on the vacuum, an electric measuring device which determines the lift variation is provided. The output signal of the measuring device is fed to the electric regulator to carry out a corresponding correction. By these measures, which are indispensable for fully satisfactory regulation, the production cost of the device undergoes another substantial increase.